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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <script>
        // 断点调试
        // 1 按f12打开开发者工具，2 带你到source一栏 3 选择代码文件
        let num = 12
        num++
        alert(num)
    </script>
</body>

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